At present, for the formation of visible images from certain image information, those methods for image forming through electrostatic latent images as in the electrophotographic process, electrostatic recording process, electrostatic printing process, and the like, are commonly used.
As the developer for use in making electrostatic latent image development there are known two types, i.e., a two component type developer comprised of a mixture of a toner and a carrier, and a one component type developer comprised of a magnetic material containing magnetic toner to be used alone without being mixed with any carrier. In an electrostatic latent image developing method which uses the former two component type developer, the toner of the developer is frictionally chargeable by mechanically stirring the mixture of it with toner carrier, so that by making an appropriate selection of the properties of the carrier and stirring condition, the polarity of the toner by charging and the amount of charge can be controlled to a considerable degree, and the developer is excellent in the developability as well as in the fluidity, and, moreover, colors, when provided to the toner are widely selectable. In this respect, the electrostatic latent image developing method using a two component type developer is considered more advantageous than that of the latter using a one-component-type developer.
Conventionally known electrostatic image developing methods which use the two-component developer are classified into two types: (1) A contact-type developing method, in which development takes place with a developer layer in direct contact with the surface of the latent image carrier, and (2) a non contact type developing method, in which a developer layer is transported, whith its surface keeping out of contact with the latent image carrier, into a developing space, and an oscillating electric field is applied to the developer layer to perform development.
According to the latter non contact type developing method, the developer layer does not come into direct contact with the latent image carrier, so that it is advantageous particularly in the multicolor image formation. That is, after forming a first color toner image on the latent image carrier, with this toner image remaining intact without being transferred, charging/exposure and developing processes for a second color are carried out, whereby the second color toner image can be superposedly formed on the first color toner image. Accordingly, it is possible to form a multicolor image easily with no need of any complex construction such as a transfer drum.
As has been mentioned above, the developing method using a two-component developer in non-contact manner in an oscillating electric field has the advantage that the developer used is satisfactory in the fluidity as well as in the frictional chargeability and excellent in the developability, and besides the selectable range of colors applicable to the toner is so wide as suitable for color copying.
In the non contact type developing method, the developing is made by flying the toner of the developer layer toward the latent image carrier, so that an oscillating electric field strong enought to fly the toner is required in the developing gap d between the latent image carrier and the developing sleeve.
The strength E of such the oscillating electric field is normally from 500 to 10,000 V/mm: the oscillating electric field, if less than 500 V/mm, is too weak to fly toner, while if more than 10,000 V/mm, is to cause a dielectric breakdown. The oscillating electric field is formed by applying an AC voltage V to the gap d between the latent image carrier and the developing sleeve, and their relationship is expressed by the formula: E=V/d(mm). If the AC voltage V in the formula comes to excess, a leak discharge is prone to occur between the developing sleeve surface and peripheral devices, inviting danger, resulting in the damage of devices or fall of bias voltage. For this reason, the foregoing AC voltage V is conventionally less than 10 KV(p--p), the developing gap d in the above formula therefore is not more than 1 mm, and the thickness of the developer layer to be transported into the developing gap d is not more than 1 mm, and more preferably not more than 0.5 mm.
There have been a number of proposals for means to obtain such a thin developer layer. For example, Japanese Utility Model Publication Open to Public Inspection (hereinafter referred to as Japanese Utility Model O.P.I. Publication) Nos. 96455/1981, 104754/1981 and 112352/1982 and Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 143650/1979 disclose a thin layer forming member having a doctor blade provided close by the sleeve surface for regulating the developer layer, in which improvement is made on the doctor blade's form and arrangement in angle to the developing sleeve, material, and the like. Also, Japanese Utility Model O.P.I. Publication No. 57866/1983 and Japanese Patent 0.P.I. Publication No. 114561/1984 propose a thin layer forming member having a magnetic blade. Japanese Utility Model O.P.I. Publication No. 86654/1983 discloses one having a sponge press on the developing sleeve to regulate the thickness of the developer layer. And Japanese Patent O.P.I. Publication No. 126567/1984 describes the use of an elastic roll to press on the sleeve surface to regulate the thickness of the developer layer.
As has been explained, various types of thin layer forming member have been proposed to date, but the layer's thickness control is not easy, tending to cause uneven thickness, which has made it difficult to steadily obtain a uniform thickness-having thin developer layer. This problem is significant particularly in the case of using a two-component developer comprising a carrier and a toner, both being different in the particle size, configuration, fluidity and other physical properties.
We had earlier proposed in our Patent Application No. 192710/1987 (Japanese Patent O.P.I. Publication No. 52566/1987) a thin layer forming member having an elastic plate to press on the developing sleeve surface to form a thin two-component developer layer. This thin layer forming member, unlike those conventional ones utilizing a fixed narrow gap, allows a wide-range elastic regulation of the layer thickness, thus enabling not only to make a reasonable layer thickness regulating operation but also to obtain uniformly, stably an even thinner developer layer. This is advantageous especially in the case where a two-component developer is used; if the above-mentioned elastic plate is arranged so as to press against the sleeve surface with a pressure allowing the passage of about 1 to 3 carrier particles, then a so thin layer in the thickness range around the carrier particle size can be easily obtained. Furthermore this is advantageous in respect that it obstructs the passage of aggregates or foreign matter contained in the developer and allows the passage of the developer layer alone.
The (a) and (b) of FIG. 1 are drawings for explaining the particular construction of a thin layer forming member which uses an elastic plate, wherein 1 is a developing sleeve which rotates in the direction of arrow, 2 is a magnetic roll having a plurality of alternate N and S polarities, 3 is a thin layer forming member consisting of an elastic plate, and 4 is a support for supporting the elastic plate 3. When the planar elastic plate 3, with its tip end facing upstream the rotating direction of the developing sleeve, presses on the surface of developing sleeve 1, a wedge-shaped space 5 is formed between the tip end and the pressed contact point. When, under this condition, developing sleeve 1 is moved in a given direction, developer D that is retained on developing sleeve 1 due to the magnetic field of magnetic roll 2 is separated into two parts: one getting into wedge-shaped space 5 and the other having failed to get into wedge-shaped space 5 and sent away toward the reverse side of elastic plate 3 opposite to developing sleeve 1, and of these parts only the developer D that has entered the wedge-shaped space passes, due to the frictional force with developing sleeve 1, through the gap between developing sleeve 1 and elastic plate 3, and then is transported to the developing region. In this instance, the amount of the developer that is allowed to pass between developing sleeve 1 and elastic plate 3 corresponds to the gap .epsilon., but is determined according to the height h of the opening of wedge-shaped space 5, free length 1, pressing force .sigma., and the like, and usually, the height h of the opening is from 0.08 to 0.3 mm, free length l is from 1 to 3 mm, and pressing force .sigma. is from 1 to 6 g/mm.
Thus, the gap .epsilon. between developing sleeve 1 and elastic plate 3 allows the passage of an amount of one to several particles of carrier, resulting in the formation of a thin developer layer with a thickness of not more than 500 .mu.m, and preferably 10 to 300 .mu.m. By using the thin layer forming member having an elastic plate of the above construction, a thin two-component developer layer having a uniform thickness can be stably obtained. Therefore, this has the advantage that non-contact development with its carrier restrained from flying can be carried out and an image excellent in the resolution can be obtained. As a result of making the developer layer a thin layer as above-mentioned, the developing gap d is allowed to be small, thus making so much the smaller the oscillating electric field and AC voltage for forming the oscillating electric field, so that the development can be carried out with no dielectric breakdown, leak discharge, etc.
Incidentally, when non-contact development is made in the oscillating electric field, if the mass of the toner on the sleeve is regarded as M and the quantity of charge as Q, there is a tendency that the smaller the Q/M, the more easily does the development take place.
Since there is a nearly inversely proportional relation between the above Q/M and the particle size of the toner, relatively large particle sizes-having toner is much more consumed and smaller particle sizes-having toner remains: thus bringing about a phenomenon called `selective development`. Accordingly, when the non-contact development is repeatedly continued, smaller particle size toner is accumulated in the developer inside the developing device to cause the carrier surfce to have lots of small particle-size toner attached thereto, bringing about such troubles as 1) fatigue/deterioration of the carrier, 2) inadequate frictional charging of the toner, 3) lowering of image density, and 4) scattering of the toner. The reason why such troubles occur is considered due to the fact that the increase in the amount of the small particle-size toner increases the surface area of the toner, causing the toner's frictional charging amount to increase to make the toner strongly attach to the carrier surface to form a coat on the carrier, deteriorating the carrier to make the subsequent frictional charging of the toner inadequate, thus lowering the image density, scattering the toner, and so forth.
On the other hand, Japanese Patent O.P.I. Publication No. 140361/1985 has already proposed a developing method which uses a two-component developer whose toner has an weight average particle size of from 5 to 20 .mu.m, and has such a particle size distribution that the more than double the weight average particle size part and the less than 1/3 of the weight average particle size part of the toner account for not more than 10% by weight of the whole toner.
According to this developing method, since the particle size distribution of the toner in the developer is limited to the above-mentioned range, the unevenness of the action of the oscillating electric field to the toner is lessened, so that the foregoing selective development phenomenon is improved.
Incidentally, according to our investigation, the selective development phenomenon is connected also with the thickness of the developer layer; the thinner the developer layer, the more significantly does the phenomenon tend to occur. That is, if the layer is thick, this phenomenon does not occur, which is considered due to the fact that the toner particles' mutual restraining action restrains the selective development.
As has been mentioned, in the case of forming a thin two-component developer layer, when the foregoing elastic plate is arranged to press on the sleeve surface, a developer layer as thin as not more than 500 .mu.m, and preferably not more than 300 .mu.m can be formed, and so much the higher resolution image can be obtained, but on the other hand, it has been found that the selective development occuring condition becomes so severe that it makes inadequate the developer as described in the foregoing Japanese Patent O.P.I. Publication No. 140361/1985 for its developing method.